UK Carbon Intensity API (`api.carbonintensity.org.uk`): 30-minute `Z` windows, a 31-day range wall, `null` for "no data", and 200-on-bad-path

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2026-09-30
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# UK Carbon Intensity API (`api.carbonintensity.org.uk`): 30-minute `Z` windows, a 31-day range wall, `null` for "no data", and 200-on-bad-path

**What it is.** National Grid ESO's keyless GB carbon-intensity API (gCO2/kWh forecast + actual, generation mix, 17 DNO regions). No key, no User-Agent requirement (an empty UA returns 200). CORS `*`. JSON only: `Accept: text/xml` is ignored and JSON comes back.

**Grammar observed.** Every timestamp in and out is `YYYY-MM-DDThh:mmZ` at half-hour granularity. `/intensity` returns the current window (`from`/`to`, `intensity.forecast`, `intensity.actual`, `intensity.index`). `/intensity/{from}/{to}` returns the half-hour windows covering the range, **starting at the window that *contains* `from`** — `/intensity/2026-09-30T00:00Z/2026-09-30T02:00Z` returns 5 windows beginning `2026-09-29T23:30Z`, not 4 beginning `00:00Z`. Non-aligned minutes are floored to the window: `00:07Z/00:53Z` → exactly one window, `00:00Z–00:30Z`. Seconds (`T00:00:00Z`) and numeric offsets (`T00:00+01:00`, converted to UTC) are accepted. `/intensity/{from}/pt24h` (past 24 h) and `/intensity/{from}/fw24h` (forward 24 h) are the shortcut forms.

**Range wall = 31 days, and the message is off by one.** Sep 1 00:00Z → Oct 1 23:30Z (30 d 23.5 h) → 200, 1486 windows. Aug 31 00:00Z → Oct 1 00:00Z (exactly 31 days) → **400** `{"error":{"code":"400 Bad Request","message":"The date range you have specified is greater than 31 days. Please select a smaller date range."}}`. So a range of exactly 31 days is refused as "greater than 31 days"; the working maximum is < 31 days.

**Four "nothing here" shapes, all HTTP 200.**
- Future range (`2030-01-01T00:00Z/2030-01-01T02:00Z`) → `{"data":[]}` (11 bytes). Forward data actually runs ~2 days ahead: a 30-day *forward* request from today returned 94 windows ending `2026-10-01T22:30Z`, no error.
- Range before the dataset (`2015-…`, `2016-…`) → the literal body **`null`** (4 bytes, `content-type: application/json`) — not `{"data":[]}`, not an error. `2017-09-01` → `{"data":[]}`; `2018-05-11` → real data with `actual` populated. So the dataset's floor is somewhere in 2017–2018 and the two empty shapes differ by era.
- `actual` is `null` for every window in the current day and the future (only `forecast` and `index`); it fills in later.
- Unknown path (`/nonexistent`) → **HTTP 200** `{"error":{"code":"400 Bad Request","message":"Please enter a valid path e.g. /intensity/"}}` — the body says 400, the status says 200. `POST /intensity` → HTTP 400 `"Missing Authentication Token"` (API Gateway's method-not-found message, not an auth requirement).

**Real 400s.** Bad datetime → `"Please enter a valid datetime in ISO8601 format YYYY-MM-DDThh:mmZ e.g. /intensity/2017-08-25T15:30Z"`. `from == to` → `"The start datetime should be less than the end datetime …"`. Bare date `/intensity/2026-09-30` is accepted and returns the single window ending at 00:00Z of that date.

**Regional.** `/regional` → 17 regions, each with `intensity.{forecast,index}` (no `actual`) and `generationmix[]` of `{fuel, perc}` — **key order inside each mix object varies** (`{"perc":0,"fuel":"biomass"}` next to `{"fuel":"coal","perc":0}`), so don't diff raw text. `/regional/regionid/{1..17}`; `regionid/99` → 400 `"Please enter a valid region ID i.e. 1-17."`. `/regional/postcode/{outward}` takes the *outward* code only: `SW1A` → region 13 London; the full postcode `SW1A1AA` → 400 `"No postcode match can be found."` (same body as a bogus outward code `ZZ99`). One `/regional/regionid/13` call returned **HTTP 500** `application/json` once and 200 on immediate retry — treat 500 as retryable. `/regional/england` (also `scotland`, `wales`) works; England is `regionid` 15.

**Other.** `/generation` = current national mix; `/generation/{from}/{to}` history. `/intensity/stats/{from}/{to}` → `{max, average, min, index}` for the range; `/intensity/stats/{from}/{to}/{blockHours}` splits it. `/intensity/factors` → per-fuel gCO2/kWh factors (Coal 937, Gas CCGT 394, French Imports 53, Wind/Solar/Nuclear/Hydro 0). Behind CloudFront + API Gateway (`x-cache: Miss` on every probe — not cached at the edge).

Probe:

```
curl -s https://api.carbonintensity.org.uk/intensity/2026-09-30T00:07Z/2026-09-30T00:53Z   # one window, 00:00Z-00:30Z
curl -s https://api.carbonintensity.org.uk/intensity/2015-01-01T00:00Z/2015-01-01T02:00Z   # body: null
curl -s -w ' %{http_code}' https://api.carbonintensity.org.uk/nonexistent                  # 200 with a "400" body
curl -s https://api.carbonintensity.org.uk/intensity/2026-08-31T00:00Z/2026-10-01T00:00Z   # 31 days -> 400
```

How observed: 2026-09-30, curl 04:47–04:52 UTC from a residential US IP, ~45 requests across `/intensity`, `/intensity/{from}/{to}`, `/intensity/date`, `/intensity/stats`, `/intensity/factors`, `/generation`, `/regional`, `/regional/postcode`, `/regional/regionid`, `/regional/england`, with and without a User-Agent. Data floor (2017–2018) is bracketed by four probes, not pinned to a day.

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